In a conventional hermetic type compressor which is used in a refrigerator, air conditioner or the like, refrigerant is compressed and the compressed refrigerant is sent to a refrigeration cycle. There are two types of refrigerants used for this purpose: CFC refrigerants and natural refrigerants. To reduce global warming impacts, the use of a natural refrigerant is desirable.
When a hydrocarbon refrigerant as a natural refrigerant is used, the cylinder capacity must be larger in order to obtain the same level of refrigerating capacity as when a CFC refrigerant is used, and in case of a closed type reciprocating compressor, the cylinder bore diameter must be larger.
As the cylinder bore diameter increases, the seal length between the piston and cylinder bore increases. This is one of the major reasons for deterioration in the hermetic type compressor performance due to compressed refrigerant leaks between the piston and cylinder bore.
One of the reasons for refrigerant leaks which occur between the piston and cylinder bore is a deformation of the cylinder bore. A cylinder bore deformation occurs when a suction valve plate, a cylinder head, a discharge valve plate, a head cover and the like are fastened and secured to the open end face of the cylinder block.
In a conventional method of reducing the cylinder bore deformation which occurs in fastening and securing the cylinder head to the cylinder block with bolts, the cylinder head is fastened to the cylinder block with bolts through a valve plate warped on both sides towards the cylinder head (see patent literature 1). Another conventional method is to provide a groove near a bolt hole in the cylinder block to prevent the bolt fastening stress on the bore (see patent literature 2).
Here, patent literature 1 refers to Japanese Patent Laid-open No. S63(1988)-230975 (FIGS. 3 to 11) and patent literature 2 refers to Japanese Patent Laid-open No. 2000-205136 (FIGS. 3, 7 and 8).
However, the conventional fastening methods have the following problems.
In the method which uses a valve plate as described in patent literature 1, the valve plate is warped and thus leakage of compressed gas easily occurs on the surface of contact between the discharge valve plate and cylinder head, leading to instability in compressor performance.
In the method as suggested in patent literature 2, where a groove is provided near a bolt hole in the cylinder block, the groove must be deep enough, resulting in deterioration in workability.